Conventionally, this type of capacitive electromagnetic flowmeter has an excitation coil for producing a magnetic field in a direction that is perpendicular to the direction of flow of the fluid that flows within the measuring tube, and a signal electrode for electrostatic capacitance coupling with a fluid that flows within the measuring tube, provided within the measuring tube, to pick up, through the signal electrode, the electromotive force that is generated in the fluid that flows within the measuring tube due to the magnetic field that is created by the excitation coil. Note that normally a guard electrode for shielding the signal electrode is provided for the signal electrode, and a pair of signal electrodes and the guide electrodes is provided in a direction that is perpendicular to the magnetic field that is produced by the excitation coil.
FIGS. 4A and 4B illustrate the critical portions of a conventional example 1 of a capacitive electromagnetic flowmeter such as proposed in U.S. Pat. No. 4,631,969. In FIGS. 4A and 4B, 1 is a measuring tube, a non-magnetic pipe 2 (such as a stainless steel pipe) having an insulating resin lining 3 on the inner peripheral surface thereof. 4 is a signal electrode, and 5 is a guard electrode for shielding the signal electrode. The signal electrode 4 and the guard electrode 5 are provided in the resin lining 3.
Note that, although not shown in FIGS. 4A and 4B, an excitation coil is provided for producing a magnetic field in a direction that is perpendicular to the direction of flow of a fluid that flows in the measuring tube 1, and two signal electrodes and 4 and guard electrodes 5 are provided facing each other in a direction that is perpendicular to the magnetic field that is produced by the excitation coil.
Illustrated in FIG. 4A and FIG. 4B, typically a fluorine resin is used as the resin lining 3. The fluorine resin has non-stick properties, so the signal electrode 4 may peel from the resin lining 3 due to vibration or by a load such as an external force, because the adhesion between the signal electrode 4 and the resin lining 3 is weak. If the signal electrode 4 were to peel off of the resin lining 3, then there would be a change in the coupling capacitance, which would have an impact on the measurement accuracy. Given this, a plurality of holes 4a is provided in the signal electrode 4, as illustrated in FIG. 5, and the adhesive force between the signal electrode 4 and the resin lining 3 is increased by connecting the resin lining 3 through the holes 4a between the front and back surfaces of the signal electrode 4. This type of structure is proposed in Japanese Utility Model Registration Application S56-137024.
However, in order to increase the adhesiveness of the fluorine resin with most metals in the structure in the second conventional example, set forth above, it is necessary to increase the diameter of the holes 4a that are formed in the signal electrode 4. When the diameter of the holes 4a is increased, then the electrode surface area of the signal electrode 4 is reduced, increasing the electrostatic capacitive impedance of the electrode portion, and thus there is a problem in that the signal is reduced.
The object of the present invention is to provide a capacitive electromagnetic flowmeter capable of increasing the adhesive force between the signal electrode and the resin lining without reducing the electrode area of the signal electrode.